The desirability of imparting a uniformily spherical surface to a work piece has long been appreciated. Exemplary of the need for such a uniformly spherical surface is the rotating flow control ball of a ball valve. Ball valves typically have a flow control ball having a substantially spherical surface and a flow control port bored therethrough. Diametrically opposed trunnions are formed with the flow control ball so as to rotate the flow control ball therein between an open and close position. Typically, the substantially spherical surface is sealably engaged by seals mounted on the ball valve body. While it is well known to manufacture such flow control balls to a substantially spherical shape, it has been found difficult to completely machine the flow control ball to a high degree of roundness. Moreover, such non-uniformity in roundness permits dirt, grime and grit to work its way between the flow control ball and the valve body or seals to scar the surface and further increase its spherical nonuniformity to damage seals and generally to increase repair frequency of such valves.
Presently it is believed that substantially spherical surfaces are imparted to ball valves and the like in a three-step process comprising machining, grinding and lapping. A roughly spherical form can be obtained by use of a lathe or the like. Next, the roughly spherical object is mounted such that it rotates about its longitudinal axis. A grinding stone is positioned in a plane containing the longitudinal axis of the object and the grinding stone is oscillated about a radius of gyration having its origin on the longitudinal axis of rotation of the object. The grinding stone oscillates within a predetermined arc to form a substantially spherical surface on the object being rotated. Finally, the object is hand lapped with a rubbing compound or the like to obtain a surface which is substantially spherical with a tolerance of 0.001 inches. The final two steps of grinding and lapping are also used to rework previously manufactured flow control balls having a worn or abraded spherical surface.